A wireless telecommunications system has been proposed in which a geographical area is divided in to cells, each cell having one or more central terminals (CTs) for communicating over wireless links with a number of subscriber terminals (STs) in the cell. These wireless links are established over predetermined frequency channels, a frequency channel typically consisting of one frequency for uplink signals from a subscriber terminal to the central terminal, and another frequency for downlink signals from the central terminal to the subscriber terminal.
Due to bandwidth constraints, it is not practical for each individual subscriber terminal to have its own dedicated frequency channel for communicating with the central terminal. Hence, techniques need to be applied to enable data items relating to different communications to be passed over the same frequency channel without interfering with each other. In current wireless telecommunications systems, this can be achieved through the use of a `Code Division Multiple Access` (CDMA) technique, whereby a set of orthogonal codes may be applied to the data items to be transmitted on a particular frequency channel, data items relating to different communications being combined with different orthogonal codes from the set.
Signals to which an orthogonal code has been applied can be considered as being transmitted over a corresponding orthogonal channel within a particular frequency channel. Hence, if a set of 16 orthogonal codes are used, 16 orthogonal channels can be created within a single frequency channel, and hence up to sixteen separate communication signals (corresponding to sixteen separate wireless links) can be transmitted simultaneously over the single frequency channel if different orthogonal codes are applied to each communication signal.
This reduces interference between signals transmitted on the same frequency channel in a particular cell, but does not avoid the prospect of interference from signals generated by nearby cells that happen to be using the same frequency channel. This problem is alleviated somewhat by adopting a deployment arrangement whereby adjacent cells all use different frequency channels. However, there are only a limited number of frequency channels that can be allocated to the wireless telecommunications system. Hence it is inevitable, particularly in densely populated areas where there are many subscriber terminals and thus each cell only covers a small geographical area, that signals from nearby cells using the same frequency channel may interfere with those generated within a cell.